Chapter 4: How Stuck Is Stuck?

If you use a reasonable adhesive such as superglue to stick some parts together, the “adhesion” doesn't just depend on the adhesive – it depends, first of all, on the test you apply to it. Testing peel, shear and butt joints gives totally different values, often differing by a factor of 1000s. This tells us that measuring the “real adhesion” is not just hard, it's impossible because adhesion is a property of the system. For example, you can change the strength of some types of bonds by changing the thickness of the bits you are sticking together. In other bonds, the strength increases as you decrease the thickness of the adhesive layer. For the lap shear joint, you can remove 60% of the adhesive with no change in strength, and, despite the name, the strength does not depend on the overlap and when it fails it does so in peel, not shear! This insistence that measured strength depends on the system isn't just nice theory – it's important in practice. Adhesive tapes are strong via polymers that are weak, your butt joint can fail from a slight mis-pull, a joint might fail from a change of material away from the bond.